My girlfriend's nipples are curved inside. sometimes they manage to come out during foreplay but immediately recede. what can be done about it?

Well,...: Some woman do not have protruding nipples. If important to you, plastic surgery could be performed at her request. Best wishes.

More foreplay?: seriously though, this could be a genetic variant. Sounds like nipple erection occurs normal, I presume. Sometimes inverted nipples are a serious concern, but less so if on both sides. If she's close to your age, she should be getting mammograms regardless. Probably nothing to worry about, but let her MD confirm. Tell her how beautiful she is and how much she is cherished, regardless. Zmd

Normal variant: I is not uncommon, and quite normal for some women to have inverted nipples that evert when excited, like during foreplay. Nothing needs to be done about them.